INSTALLATION MANUAL
CMA-5024 GLSSU
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without prior written permission by CMC Electronics Inc.
Page 22
November 21, 2008
K. GLSSU operating modes
The GLSSU selects automatically and operates in one of the following modes starting from application
of DC power. However transition to/from SBAS PA mode depends on data received over the ARINC
429 busses. Depending on the APD Configuration, approach path parameters or an external control
command may be required. The actual mode of operation is broadcast over the ARINC 429 output
busses on label 273; refer to Appendix C for the data format:
Self-Test
: The GLSSU performs extensive BIT tests at power-up for ten seconds.
Initialization:
Following self-test, the GLSSU initializes briefly its hardware prior to entering
the Acquisition mode.
Acquisition
: The GLSSU starts searching for GPS and SBAS satellite signals. Once a
satellite is detected, its signals are tracked and ranging measurements are
made. However, there are yet not enough satellites tracked for computing a
Position/Velocity/Time (PVT) navigation solution.
Navigation (NAV):
The GLSSU tracks at least four GPS satellite signals and optionally altimeter
data (after internal altimeter calibration) for computing a valid PVT solution, ten
times per second. The GLSSU monitors the integrity of the PVT solution using
a RAIM algorithm, suitable for enroute aircraft navigation through LNAV
approaches.
SBAS Navigation (SBAS NAV)
: The GLSSU tracks both GPS and SBAS satellite signals for
computing an improved PVT solution, ten times per second. The GLSSU
monitors the integrity of the PVT solution using the integrity data received from
SBAS satellites, suitable for enroute aircraft navigation through LNAV
approaches.
SBAS PA
: The GLSSU tracks both GPS and SBAS satellite signals for computing an
improved PVT solution, ten times per second. The GLSSU monitors the
integrity of the PVT solution using the integrity data broadcasted by the SBAS
satellites. The process is similar to the SBAS Navigation mode but providing
higher integrity levels so that the PVT solution and guidance data are suitable
for SBAS LNAV/VNAV, LP, and LPV approaches.
Altitude Aiding
: Degraded mode of operation. Only three GPS/SBAS satellites are being
tracked. However an optional altitude input is available, allowing the GLSSU to
compute a coarse PVT solution, after internal altimeter calibration. This mode
speeds the recovery back to the Navigation mode.
Aided
: Degraded mode of operation. The Altitude Aiding mode is not possible. The
GLSSU computes a coarse PVT solution by dead-reckoning using available
track, heading and velocity data inputs. This mode speeds the recovery back to
the Navigation mode.
Fault:
The GLSSU outputs are degraded due to internal fault(s) that render the unit
unable to output reliable navigation data. The GLSSU stops tracking satellites
and the FAULT output discrete signal is activated (closure to ground).
Data Load
: (Non operational mode) The GLSSU allows reprogramming through a
dedicated RS-232 serial interface bus or the ARINC 429 bus (dedicated ARINC
615 DLU input bus and any GLSSU output bus) for shop maintenance
purposes only. Using this mode, monthly SBAS approach database uploading
can be performed on aircraft while on the ground.
The document reference is online, please check the correspondence between the online documentation and the printed version.
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